Several issues ago my friend Carl Deul referred to my use of Turface,* along with peat moss, as a potting medium for tropical rhododendrons. The subject warrants further discussion.
Turface is made by firing Mississippi montmorillionite at some 700° Celsius (1,300°F.), with subsequent grinding and screening. It is used primarily for nurturing golfing tees and greens, hence the name. I am told that Sorbalite and certain brands of cat litter are similar products, but have not tried them. Turface is available in various grits; the one I use resembles a fairly coarse gravel. Turface costs about 150 per liter, com-pared with 5C for Perlite. With valued plants and after mixing with an organic component, this threefold difference becomes insignificant.
After drenching, Turface will hold more than twice as much water as Perlite.
In contrast to its predominant rival in soilless mixes - Perlite - Turface has the following virtues: (1) cation exchange capacity - i.e., your fertilizer doesn't leach out so fast when it rains; (2) no dead air cells & is thus more absorbent and less flotational; (3) Turface is denser, lowering the center of gravity so as to make often leggy vireyas less tipsy under gusty conditions; (4) the color blends nicely with that of peat moss.
For small rhododendrons I find that a 2:1 ratio of Turface to peat moss is best, while with larger specimens 1:1 is satisfactory. As the roots grow the organic component decays, enhancing drainage.
Further to improve aeration, I use chopped or crushed bracken fronds at the container's bottom. Bracken is common worldwide and holds up remarkably well in plant pots. The use of bracken was suggested by Mr. E.F. Allen, of Copdock, Suffolk, England.